The invention relates generally to a live spindle lathe in combination with a rotary table for positioning a workpiece for machining purposes and, more particularly, to a method and apparatus for calibrating such an apparatus for on-machine measuring of workpiece features produced on the apparatus. On-machine inspection is used in production to eliminate the time and expense of an additional operation to inspect a machined component (workpiece). It allows the workpiece to be easily re-machined when required and in fact, measurement and machining iterations can be designed into the production process with little added expense and greatly improved quality results. While usually workpieces are measured manually on the machine tool or removed to a separate measurement machine for inspection purposes, business conditions or large workpiece size may dictate automatic on-machine inspection using a measurement apparatus.
Automation of on-machine inspection is a good solution for large workpieces and it can eliminate the human error factor for even small workpieces. This means the machine itself is used as a measurement tool. The cutter is replaced with an electronic device called a touch probe. The touch probe is capable of sending a signal to the machine controller when it touches a surface. The controller then records the position of the probe. When two surface locations are recorded by this means, the difference in probe positions recorded constitutes a dimension measurement.
On-machine, automatic inspection has been used for years on lathes and machining centers. Lathes have two linear axes (i.e. the z and the x axes) and a workholding spindle. The spindle holds and turns the workpiece while the two linear axes move and position the cutting tool in a two dimensional plane. Therefore, normal lathes are limited to producing rotationally symmetric features. When the cutting tool is replaced with the touch probe, the movement is also confined to a two dimensional plane. Therefore, the lathe is usually limited to measuring diameters, wall thicknesses, flange thicknesses, and lengths of workpieces. Machining centers usually at least have three linear axes (X, Y, and Z). They can be used to make and measure 3-dimensional, non-rotationally symmetric features.
Recently, lathes have been designed with “live spindles” to hold and rotate a cutting tool such a drill or endmill while positioning a rotary table to angularly position the workpiece. These lathes can perform tasks that traditionally required a machining center in addition to the normal lathe function when the live spindle is replaced with a stationary cutting tool and the rotary table spins the component. Therefore, they allow certain operations to be combined on a single machine for added production efficiency. The automatic inspection of the three dimensional, non rotationally symmetric features on a lathe is a new challenge and requirement that has arisen as a result of this new type of machine.